Energetics Flashcards
What is there usually when there a chemical reaction happens?
Change in energy
Enthalpy change
Define Enthalpy change
∆H (delta H) of the heat energy transferred in a reaction at constant pressure. The units of ∆H are kJ mol-1
Ho of Enthalpy change means
Ho of Enthalpy change
Substances were in standard states and measurement was made under standard conditions
Standard conditions for Enthalpy change are?
Standard conditions are 100kPa pressure
298K
Stated temperature
ΔH298
Describe exothermic
Give out energy
ΔH is negative
Describe endothermic
Takes in energy
ΔH is positive
Exothermic reactions are usually
Oxidation e.g. Combustion of a fuel like methane CH4(g) + O2 (g) → CO2(g) + H2O(l) ΔcH=-890 kJ mol-1 Oxidation of carbohydrates eg. C6H12O6 in respiration
Endothermic examples
Thermal decomposition of calcium carbonate
CaCO3(s)-> CaO(s) + CO2(g) ΔH=+178kJmol-1
Main reactions of photosynthesis
In exothermic reactions what Happens to temperature
Increase
In endothermic reactions what Happens to temperature
Decrease
What happens in bonds when a reaction happens?
Reactant bonds are broken and product bonds are formed
What do you need to break bonds?
Energy so bond breaking is endothermic ΔH is positive
Stronger bonds take more energy to break
What happens when bonds are formed?
Energy is released so bond making is exothermic ΔH if negative
Stronger bonds release more energy when they form
What’s Enthalpy change for a reaction
The overall sum of these two changes
If you need more energy bonds then released when bonds made ΔH if positive
If less ΔH is negative
How do you get mean enthalpies do?
Up in data books
What’s bond Enthalpy?
The energy required to break bonds
When you break the same type of bond does it always require the same amount of energy?
No the energy needed to beak a bond depends on the environment its in
What’s mean bond Enthalpy?
The average energy needed to break a certain type of bond over a range of compounds
Example of mean bond Enthalpy
Water has two O-H bonds
First bond Enthalpy= +492kJ mol-1
Second bond Enthalpy= +428kJmol-1
Mean bond Enthalpy 492+428/2=+460kJmol-1
Bond enthalpies are?
Always positive because breaking bonds is always endothermic
What happens in a chemical reaction during bond formation?
Energy absorbed to break bonds and given out during bond formation
Enthalpy change=
Enthalpy change=
Total energy absorbed-total energy released
What is so because bond energies are averages?
Enthalpy changes aren’t exact
They are slightly less accurate than using Hess’s law
Define standard Enthalpy of formation?
ΔfHo
Is the Enthalpy change when 1 mole of compound is formed from its elements in their standard states
Define standard Enthalpy of combustion
ΔcHo is the Enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions
What can you find out using calorimetry?
Enthalpy change
How can you use calorimetry to find out how much heat is given out by a reaction?
By measuring a temperature
What do you use to find the enthalpy combustion of a flammable liquid? Step 1
You burn it inside a calorimeter?
What do you use to find the enthalpy combustion of a flammable liquid? Step 2
As fuel burns it heats the water
You can work out heat energy that has been absorbed by water if you know the mass of water the temperature change and the specific heat capacity of water (4.18Jg-1K-1)
What would ideally happen in calorimetry?
The heat given out by the fuel as it burns would be absorbed by the water allowing you to work out the enthalpy change of combustion exactly
What actually happens in calorimetry?
You always lose some heat so hard to get really accurate result
Formula to calculate enthalpy change from a calorimetry experiment?
q=mcΔT
Q= heat lost or gained in joules same as enthalpy change if pressure is constant
M= mass of water/ other solution in calorimeter (g)
C= specific heat capacity of water (4.18Jg-1K-1)
ΔT= change in temperature (K) of water/ solution
What’s specific heat capacity of water?
The amount of heat energy it takes to raise the temperature of 1g by 1K
The official ΔT units are K but what are they equal to for change in temperature?
oC
In a laboratory experiment, 1.16g of an organic liquid fuel were completely burned in oxygen. The heat formed during the combustion raised the temperature ΔcHo of the fuel. Mr is 58.0
Step 1
Calculate amount of heat capacity using q=mcΔT
q= 100 x 4.18 x (358-295)= 26334J
In a laboratory experiment, 1.16g of an organic liquid fuel were completely burned in oxygen. The heat formed during the combustion raised the temperature ΔcHo of the fuel. Mr is 58.0
Step 2
Standard enthalpies are always given in kJ mol-1
Change units of q from joules to kilojoules (26334/1000)= 26.334kJ
In a laboratory experiment, 1.16g of an organic liquid fuel were completely burned in oxygen. The heat formed during the combustion raised the temperature ΔcHo of the fuel. Mr is 58.0
Step 3
Standard enthalpy of combustion is energy produced by burning 1 mole of fuel. So find out how many moles of girl produced this much energy using number of moles= mass/Mr
Number of moles of fuel= 1.16/ 58=0.0200
In a laboratory experiment, 1.16g of an organic liquid fuel were completely burned in oxygen. The heat formed during the combustion raised the temperature ΔcHo of the fuel. Mr is 58.0
Step 4
Heat produced by 1 mole of fuel =-26.334/0.0200=-1316.7kJmol-1
-1320kJmol-1
What can calorimetry also be used for?
Calculate enthalpy change for a reaction that happens in solution e.g. Neutralisation, dissolution (dissolving) or displacement
How do you find the enthalpy for a neutralisation reaction?
Add known volume of acid to insulated container (e.g. Polystyrene cup) and measure the temperature
Add known volume of alkali and record temperature change (stir solution to make sure it’s evenly heated)
Work out heat needed to raise temperature of solution q=mcΔT this equals the heat given out by the reaction
What happens if the alkali is solid?
Known mass
What happens if you’re mixing two solutions?
Need to include masses of both
What can you assume about all liquids?
Have same density as water since 1cm3 leg water has mass of 1g
E.g. 50cm3 of solution
Mass=50g for q=mcΔT
What do you need to go if you are trying to find the energy change per mole of reactant?
Moles= concentration (mol dm-3) x volume(dm3)
To find number of moles of substance in a solution
Experimental problems with all calorimetry?
Some heat will be absorbed by container rather than going towards heating up the water
Some heat always lost to surroundings during experiment (however well the containers insulated)
Experimental problems that only apply to flammable liquid calorimetry?
Some combustion may be incomplete meaning less energy will be given out
Some flammable liquid may escape by evaporation (usually quite volatile)
Hess’s law?
Total enthalpy change of a reaction is independent of the route taken
What does ΔrH mean?
The enthalpy change of a reaction
What’s Hess’s law good for?
Working out enthalpy changes that you can’t find directly by doing an experiment
Enthalpy changes of formation are useful for?
Calculating enthalpy change you can’t find directly
What do you need to know to use Hess’s law?
ΔfHo for all reactants and products that are compounds
ΔfHo for all elements is zero elements being formed from element so no change
For formations ΔrHo?
The sum of ΔfHo (products) - the sum of ΔfHo (reactants)
If big 2 multiple by 2 to get sum
What are standard enthalpy changes measured at?
298K
What happens if you go backwards along an arrow in Hess’s law diagram?
Substraction enthalpy change that goes with that arrow
E.g. Could use ΔfHo for ethanol using these routes instead
